Catalyst system for the reduction of NOx and NH3 emissions

ABSTRACT

This catalyst system simultaneously removes ammonia and enhances net NOx conversion by placing an NH 3 —SCR catalyst formulation downstream of a lean NOx trap. By doing so, the NH 3 —SCR catalyst adsorbs the ammonia from the upstream lean NOx trap generated during the rich pulses. The stored ammonia then reacts with the NOx emitted from the upstream lean NOx trap—enhancing the net NOx conversion rate significantly, while depleting the stored ammonia. By combining the lean NOx trap with the NH 3 —SCR catalyst, the system allows for the reduction or elimination of NH 3  and NOx slip, reduction in NOx spikes and thus an improved net NOx conversion during lean and rich operation.

BACKGROUND OF INVENTION

[0001] 1. Field of the Invention

[0002] The present invention is directed to a catalyst system tofacilitate the reduction of nitrogen oxides (NOx) and ammonia from anexhaust gas. More particularly, the catalyst system of this inventionincludes a lean NOx trap in combination with an ammonia selectivecatalytic reduction (NH₃—SCR) catalyst, which stores the ammonia formedin the lean NOx trap during rich air/fuel operation and then reacts thestored ammonia with nitrogen oxides to improve NOx conversion tonitrogen when the engine is operated under lean air/fuel ratios. In analternate embodiment, a three-way catalyst is designed to producedesirable NH₃ emissions at stoichiometric conditions and thus reduce NOxand NH₃ emissions.

[0003] 2. Background Art

[0004] Catalysts have long been used in the exhaust systems ofautomotive vehicles to convert carbon monoxide, hydrocarbons, andnitrogen oxides (NOx) produced during engine operation intonon-polluting gases such as carbon dioxide, water and nitrogen. As aresult of increasingly stringent fuel economy and emissions standardsfor car and truck applications, it is preferable to operate an engineunder lean conditions to improve vehicle fuel efficiency and lower CO₂emissions. Lean conditions have air/fuel ratios greater than thestoichiometric ratio (an air/fuel ratio of 14.6), typically air/fuelratios greater than 15. While lean operation improves fuel economy,operating under lean conditions increases the difficulty in treatingsome polluting gases, especially NOx.

[0005] Regarding NOx reduction for diesel and lean burn gasoline enginesin particular, lean NOx adsorber (trap) technologies have been widelyused to reduce exhaust gas NOx emissions. Lean NOx adsorbers operate ina cyclic fashion of lean and rich durations. The lean NOx trap functionsby adsorbing NOx when the engine is running under lean conditions—untilthe NOx trap reaches the effective storage limit—followed by NOxreduction when the engine is running under rich conditions.Alternatively, NOx reduction can proceed by simply injecting into theexhaust a sufficient amount of reductant that is independent of theengine operation. During this rich cycle, a short rich pulse ofreductants, carbon monoxide, hydrogen and hydrocarbons reduces the NOxadsorbed by the trap during the lean cycle. The reduction caused duringthe rich cycle purges the lean NOx adsorber, and the lean NOx adsorberis then immediately available for the next lean NOx storage/rich NOxreduction cycle. In general, poor NOx reduction is observed if the airexcess ratio λ is above 1. NOx reduction generally increases over leanNOx adsorbers as the λ ratio is decreased lower than 1. This air excessor lambda ratio is defined as the actual air/fuel ratio divided by thestoichiometric air/fuel ratio of the fuel used. The use of lean NOxadsorber (trap) technology, and in particular the rich pulse ofreductants, can cause the λ ratio to reach well below 1.

[0006] Lean NOx traps, however, often have the problem of low NOxconversion; that is, a high percentage of the NOx slips through the trapas NOx. NOx slip can occur either during the lean portion of the cycleor during the rich portion. The lean NOx slip is often called “NOxbreakthrough”. It occurs during extended lean operation and is relatedto saturation of the NOx trap capacity. The rich NOx slip is oftencalled a “NOx spike”. It occurs during the short period in which the NOxtrap transitions from lean to rich and is related to the release ofstored NOx without reduction. Test results depicted in FIG. 1a haveshown that during this lean-rich transition, NOx spikes, the large peaksof unreacted NOx, accounts for approximately 73% of the total NOxemitted during the operation of a lean NOx trap. NOx breakthroughaccounts for the remaining 27% of the NOx emitted.

[0007] An additional problem with lean NOx traps arises as a result ofthe generation of ammonia by the lean NOx trap. As depicted in FIG. 1b,ammonia is emitted into the atmosphere during rich pulses of the leanNOx adsorber. In laboratory reactor experiments, ammonia spikes as highas 600 ppm have been observed under typical lean NOx adsorber operation(see FIG. 1b). While ammonia is currently not regulated, ammoniaemissions are being closely monitored by the U.S. EnvironmentalProtection Agency; and, therefore, reduction efforts must be underway.Ammonia is created when hydrogen or hydrogen bound to hydrocarbonsreacts with NOx over a precious metal, such as platinum. The potentialfor ammonia generation increases for a precious metal catalyst (such asa lean NOx trap) as the λ ratio is decreased, as the duration of therich pulse increases, and the temperature is decreased. There is thus anoptimum lambda and rich pulse duration where the maximum NOx reductionis observed without producing ammonia. Attempts to enhance conversion ofNOx by decreasing the λ ratio of the rich pulse duration leads tosignificant production of ammonia and thus results in high gross NOxconversion (NOx→N₂+NH₃), but much lower net NOx conversion (NOx→N₂).

[0008] In addition to nitrogen, a desirable non-polluting gas, and theundesirable NH₃ described above, N₂O is another NOx reduction products.Like NH₃, N₂O is generated over NOx adsorbers and emitted into theatmosphere during rich pulses. The gross NOx conversion is the percentof NOx that is reduced to N₂, N₂O and NH₃. The net NOx conversion is thepercent of NOx that is reduced to nitrogen, N₂, only. Accordingly, thegross NOx conversion is equal to the net NOx conversion if nitrogen isthe only reaction product. However, the net NOx conversion is almostalways lower than the gross NOx conversion. Accordingly, a high grossNOx conversion does not completely correlate with the high portion ofNOx that is reduced to nitrogen.

[0009] The NOx conversion problem is magnified for diesel vehicles,which require more than a 90% NOx conversion rate under the 2007 U.S.Tier 11 BIN 5 emissions standards at temperatures as low as 200° C.While high NOx activity is possible at 200° C., it requires extrememeasures such as shortening the lean time, lengthening the rich purgetime, and invoking very rich air/fuel ratios. All three of thesemeasures, however, result in the increased formation of NH₃ or ammonia.Accordingly, while it may be possible to achieve 90+% gross NOxconversion at 200° C., to date there has not been a viable solution toachieve 90+% net NOx conversion.

[0010] Accordingly, a need exists for a catalyst system that eliminatesNOx breakthrough during the lean operation as well has the NOx spikesduring the lean-rich transition period. There is also a need for acatalyst system that is capable of improving net NOx conversion.Finally, there is a need for a catalyst system capable of reducingammonia emissions.

SUMMARY OF INVENTION

[0011] This invention provides a solution for all of the above problemsand, in particular, reduces or eliminates ammonia emissions and improvesthe net NOx conversion of the catalyst system. These problems are solvedby simultaneously removing ammonia and enhancing NOx conversion with theuse of an NH₃—SCR catalyst placed downstream of the lean NOx adsorbercatalyst, as shown in FIG. 2. The NH₃—SCR catalyst system serves toadsorb the ammonia emissions from the upstream lean NOx adsorbercatalyst generated during the rich pulses. Accordingly, as shown in FIG.2, the ammonia emissions produced by the lean NOx adsorber is stored andeffectively controlled by the NH₃—SCR catalyst rather than beingemitted. This reservoir of adsorbed ammonia then reacts directly withthe NOx emitted from the upstream lean NOx adsorber. As a result, asshown in FIG. 3, the overall net NOx conversion is enhanced from 55% to80%, while depleting the stored ammonia, as a function of the SCRreaction: NH₃+NOx→N₂. The NH₃—SCR catalyst is then replenished withammonia by subsequent rich pulses over the lean NOx adsorber.

[0012] During the lean cycle for this lean NOx adsorber+NH₃—SCR system,the NOx breakthrough from the upstream lean NOx adsorber is reducedcontinuously as it passes over the NH₃—SCR until the reservoir ofammonia is depleted. In addition, during the rich cycle, large spikes ofunreacted NOx are created. The downstream NH₃—SCR catalyst thus servesto dampen these large NOx spikes by reacting the unreacted NOx with thereservoir of stored ammonia emitted from the lean NOx adsorber. Ingeneral, the combination of the lean NOx adsorber+NH₃—SCR catalystsystem allows for the reduction, or elimination, of ammonia emissionsand NOx slip, i.e., reduction of NOx breakthrough and NOx spikes and,therefore, improved net NOx conversion during lean and rich operation.

[0013] Additionally, under this invention, urea and/or ammonia does notneed to be injected into the exhaust system to effectuate the reactionbetween NOx and ammonia. Rather, the ammonia is automatically generatedfrom the NOx present in the exhaust gas as it passes over the preciousmetal lean NOx adsorber during the rich pulses. The generated ammonia isthen stored on the downstream NH₃—SCR catalyst, to react with theunreacted NOx, and thereby convert the unreacted NOx to nitrogen.

[0014] The NH₃—SCR catalyst thus serves to adsorb the ammonia from theupstream lean NOx adsorber catalyst generated during the rich pulses.Under this system, the ammonia is stored and effectively controlledrather than being emitted. This reservoir of adsorbed ammonia thenreacts directly with any NOx emitted from the upstream lean NOxadsorber. As a result, the overall net NOx conversion is enhanced from55% to 80%, while the overall gross NOx conversion is enhanced from 68%to 82%, as shown in FIG. 3.

[0015] In one alternative embodiment of this invention, the catalystsystem can be optimized and NOx reduction increased by verticallyslicing the lean NOx trap and NH₃—SCR catalyst substrates to createseparate catalyst zones, such that the catalytic converter shell or canwould have alternating sections of lean NOx trap and NH₃—SCR catalysts,as shown in FIG. 4. Under this embodiment, both technologies, the leanNOx trap formulation and the NH₃—SCR formulation, can be incorporatedinto a single substrate and/or a single converter can rather thanplacing the NH₃—SCR catalyst downstream of the lean NOx adsorber as twoseparate and distinct catalyst substrates.

BRIEF DESCRIPTION OF DRAWINGS

[0016]FIG. 1a is a graph illustrating the NOx spikes that occur duringthe NOx trap lean-rich transition;

[0017]FIG. 1b is a graph illustrating NOx and NH₃ emissions from atypical prior art lean NOx adsorber system;

[0018]FIG. 2 depicts the lean NOx and NH₃—SCR catalyst system of thepresent invention;

[0019]FIG. 3 depicts reduced NOx emissions and NH₃ emissions as a resultof the use of the lean NOx and NH₃—SCR catalyst system of the presentinvention, as shown in FIG. 2;

[0020]FIG. 4 depicts three different zoned catalyst embodiments of thelean NOx and NH₃—SCR catalyst system;

[0021]FIG. 5 is a graph illustrating the reduced levels of NOx and NH₃emissions resulting from each of the three zoned catalyst embodimentsdepicted in FIG. 4 at a 250° C. inlet gas temperature and operating at a50 second lean cycle and 5 second rich cycle;

[0022]FIG. 6 is a graph illustrating the reduced levels of NOx and NH₃emissions resulting from each of the three zoned catalyst embodimentsdepicted in FIG. 4 at a 200° C. inlet gas temperature and operating at a25 second lean cycle and a 5 second rich cycle;

[0023]FIG. 7 shows three proposed examples of washcoat configurationsincorporating the lean NOx trap and NH₃—SCR formulations into the samesubstrate;

[0024]FIG. 8 is a graph illustrating the impact of NOx conversion afterhydrothermal aging; and

[0025]FIG. 9 depicts a modified three-way catalyst and NH₃—SCR catalystsystem of the present invention.

DETAILED DESCRIPTION

[0026] In this invention, net NOx conversion is improved and ammoniaemissions reduced through the use of a lean NOx trap and NH₃—SCRcatalyst system which operate together to produce and store ammonia andreduce NOx to nitrogen. In so doing, the catalyst system of the presentinvention solves three problems of lean NOx traps; namely, reducing NOxbreakthrough, NOx spikes and ammonia emissions.

[0027] In order to meet increasingly stringent fuel economy standards,it is preferable to operate an automotive engine under lean conditions.However, while there is improvement in fuel economy, operating underlean conditions has increased the difficulty in reducing NOx emissions.As an example, for a traditional three-way catalyst, if the air/fuelratio is lean even by a small amount, NOx conversion drops to lowlevels. With traditional three-way catalysts, the air/fuel ratio must becontrolled carefully at stoichiometric conditions to maximize reductionof hydrocarbons, carbon monoxide and NOx.

[0028] Throughout this specification, NOx refers to nitrogen oxides,which include nitrogen monoxide NO and nitrogen dioxide NO₂. Further,lean NOx adsorber and lean NOx trap are used interchangeably throughoutthis specification.

[0029] To achieve NOx reduction, under lean operating conditions, oneoption is the inclusion of a lean NOx trap. While the lean NOx trap isgenerally effective in NOx reduction, lean NOx traps are known to havethe problems referred to as “NOx slip” which includes breakthrough ofNOx during the extended lean operation of the NOx trap and also NOxspikes generated during the transition from the lean to the rich cycle.

[0030] NOx spikes, or NOx emissions during the lean-rich transition, arebelieved to occur due to the exothermic heat generated from theoxidation of reductants, carbon monoxide, hydrocarbons and hydrogen, bythe oxygen released from the oxygen storage material—the temperaturerise can be as high as 80-100° C.

[0031] The problem of NOx spikes is illustrated in FIG. 1a, and theproblem of insufficient net NOx conversion is illustrated in FIG. 1b.FIG. 1b depicts laboratory reactor data of a lean NOx adsorber systemoperating in an 85 second lean and 5 second rich cyclic pattern. Theplot in FIG. 1b shows the nitrogen species concentration as a functionof time. The laboratory reactor data depicted in FIG. 1b resulted from acatalyst having an engine swept volume (ESV) of 100%. Additionally, thereactor used to obtain the results in FIG. 1b was at a temperature of300° C. To begin the cycle, 500 ppm of nitrogen oxide was fed into thereactor where much of it was stored during the 85 second lean duration.During the 5 second rich duration, nitrogen oxide was reduced; however,a significant amount of ammonia was formed. As illustrated in FIG. 1b,the data shows ammonia spikes as high as 600 ppm under typical lean NOxadsorber operation. Conversion, however, is generally improved as the λratio is decreased during the rich pulse. Decreasing the λ ratio alsoleads to significant production of ammonia and thus results in highgross NOx conversion (NOx→N₂+NH₃), but much lower net NOx conversion(NOx→N₂). As illustrated in FIG. 1b, the net NOx conversion to nitrogenfor this lean NOx adsorber system was only 55%.

[0032] Under the catalyst system of this invention, ammonia is reducedand the net NOx conversion improved simultaneously by placing an NH₃—SCRcatalyst formulation downstream of the lean NOx adsorber catalyst, asshown in FIG. 2.

[0033]FIG. 2 is an illustration of the catalyst system of thisinvention, which is capable of simultaneously eliminating ammoniaemissions and improving net NOx conversion. As illustrated in FIG. 2,NOx produced during engine operation is stored by the lean NOx adsorberduring the lean cycle. Following the lean cycle, during the rich cycleof the lean NOx adsorber, NOx is reduced and ammonia generated. The leanNOx adsorber stores much of the NOx during the lean operation and thenreduces NOx during rich pulses of the reductants. During the same richpulses, significant amounts of ammonia are generated, as furtherillustrated in FIG. 1. As illustrated in FIG. 2, the lean NOx adsorberemits NO, NO₂, NH₃, and N₂O. These same gases then pass through theNH₃—SCR, where NH₃ is stored. Accordingly, the addition of the NH₃—SCRcatalyst downstream allows for the adsorption of NH₃ and subsequentreaction with any NOx that slips through the upstream lean NOx adsorber,which thus improves the overall net NOx conversion (NH₃+NO→N₂). As canbe seen in FIG. 2, the catalyst system of this invention results in asignificant net NOx conversion improvement, the elimination of ammoniaemissions, and the production of non-polluting gases nitrogen and N₂O.

[0034] It should be noted that for diesel applications, lean NOxadsorbers must operate at lower temperatures compared to gasoline leanNOx adsorbers since the exhaust temperatures of diesel engines aresignificantly lower. More ammonia is generated at 200° C. than at 300°C. over lean NOx adsorbers, and thus the catalyst system of thisinvention has an even greater potential for diesel applications.Likewise, the problem of NOx spikes is more critical at highertemperatures, the temperatures used for gasoline applications; and thusthe catalyst system of this invention is beneficial to control theunreacted NOx spikes that result from the operation of a lean NOxadsorber at operating temperatures typical for gasoline lean NOxadsorber applications.

[0035] The NH₃—SCR catalyst thus serves to adsorb the ammonia producednaturally from the upstream lean NOx adsorber catalyst generated duringthe rich pulses. As a result, the NH₃—SCR catalyst stores the ammonia,controlling it rather than allowing it to be emitted into theatmosphere. This reservoir of adsorbed NH₃ in the NH 3—SCR catalystreacts directly with the NOx emitted from the upstream lean NOx adsorber(trap).

[0036] In general, this invention works to clean NOx emissions—and thushas applicability for stationary sources as well as for moving vehicles.This invention may be used to reduce NOx emissions for nitric acidplants, or any other stationary source that requires the reduction ofNOx emissions. This invention is nonetheless particularly directed foruse with gasoline and diesel vehicles which, unlike stationary sources,have a wide range of operating parameters, especially temperatureparameters—which cannot be precisely controlled. The present inventionhas the ability to store large quantities of ammonia across a broadtemperature range to effectuate the reaction between ammonia andnitrogen oxides and thereby convert NOx to nitrogen.

[0037] As illustrated in FIG. 3, laboratory experiments havedemonstrated that the use of a lean NOx adsorber plus NH₃—SCR catalystsystem improves net NOx conversion from 55%, as illustrated in FIG. 1,to 80%. FIG. 3 is a graph displaying laboratory data obtained using thecatalyst system of this invention, wherein NOx ppm are charted as afunction of time. As illustrated in FIG. 3, the catalyst system of thisinvention completely eliminated the ammonia spikes created during therich pulses of the lean NOx adsorber. In this system, ammonia is storedon the NH₃—SCR catalyst where it reacts with NOx during the 85 secondlean duration, which thus improves the net NOx conversion from 55% to80% with no additional fuel economy penalty. As shown in FIG. 3, theimproved net NOx conversion can be observed by the much narrowerprofile—zero ppm NOx is emitted for a significant amount of time ascompared to the graph shown in FIG. 1 of a system lacking theNH₃—SCR+lean NOx adsorber combination.

[0038] The reaction between the stored ammonia and NOx increases theoverall net NOx conversion, which is enhanced from 55%—the amount of NOxconverted in prior art lean NOx trap systems—to 80%—as a result of thecombination of a lean NOx trap and NH₃—SCR catalyst system. Moreover, inaddition to improving net NOx conversion, the ammonia stored in theNH₃—SCR catalyst is depleted during the SCR reaction wherein ammonia andnitrogen oxide are reacted to produce nitrogen. The NH₃—SCR catalyst isreplenished with ammonia by subsequent rich pulses over the lean NOxadsorber that causes a portion of the NOx to react with hydrogen to formammonia.

[0039] It should be noted that no urea or ammonia needs to be injectedinto the exhaust system to effectuate the reaction between ammonia andNOx. Rather, the ammonia is naturally generated from the NOx present inthe exhaust gas as it passes over the lean NOx trap during rich pulses.More specifically, ammonia is naturally created during the fuel richcycle of the lean NOx trap. Ammonia is naturally produced as it passesover the precious metal active component of the lean NOx trap.Similarly, the ammonia could also be generated in a conventionalprecious metal based TWC located upstream of a LNT/NH_—SCR system.

[0040] For this invention, the lean NOx trap is optimized for ammoniageneration by removing oxygen storage capacity (OSC) and therebyenhancing the rich cycle, and thus creating a greater quantity ofammonia for reaction with the NOx in the downstream NH₃—SCR catalyst. Ina preferred embodiment, the lean NOx trap includes platinum as theprecious metal. Platinum is the preferred precious metal because it isbelieved that a greater quantity of NH₃ is produced over platinum thanrhodium, palladium and/or a combination of the precious metals.Nonetheless, other precious metals such as palladium and rhodium, andthe combination of one or more of the precious metals platinum,palladium and rhodium may also be used to generate NH₃.

[0041] Additionally, the lean NOx trap of this invention preferablyincludes a “NOx adsorbing material” or NOx storage component/material,which can be alkali and alkali earth metals such as barium, cesium,and/or rare earth metals such as cerium and/or a composite of cerium andzirconium. Although an alternative catalyst formulation that does notcontain a NOx storage component but generates ammonia from NOx may alsobe utilized, in the most preferred embodiment, the NOx storage materialshould have the ability to store NOx at low temperature ranges,specifically in the range of 150° C.-300° C. The NH₃ thermodynamicequilibrium under rich conditions is maximized during the temperaturerange of 150° C.-300° C.

[0042] In general, to increase the NOx storage function of the lean NOxtrap and effectuate the NOx conversion reaction, in the preferredembodiment, the lean NOx trap has the following characteristics: (1) theinclusion of platinum as the precious metal; (2) the ability to storeNOx between 150° C. and 500° C. during the lean portion of the cycle;(3) the ability to maximize the duration of the lean NOx trap richcycle; (4) the ability to generate ammonia at the 150° C.-500° C.temperature range; (5) minimize OSC to lessen fuel penalty; and (6)lower lambda to generate more ammonia. Ammonia production is maximizedat the preferred temperature range, 150° C.-300° C.—which alsocorrelates with the steady state equilibrium range for ammonia creation.It bears emphasis that other NOx storage components may be utilized,especially for stationary sources, where sulfur poisoning does not posea threat.

[0043] Most simply, the NH₃—SCR catalyst may consist of any material orcombination of materials that can adsorb ammonia and facilitate theNOx+NH₃ to yield nitrogen. The NH₃—SCR catalyst should preferably bemade of a base metal catalyst on a high surface area support such asalumina, silica, titania, zeolite or a combination of these. Morepreferably, the NH₃—SCR catalyst should be made of a base metal selectedfrom the group consisting of Cu, Fe and Ce and/or a combination of thesemetals, although other base metals may be used. Base metals generallyare able to effectuate NOx conversion using ammonia while both the basemetals and the high surface support material serves to store NH₃. Thebase metal and high surface area support such as zeolite selected shouldpreferably be one that can store NH₃ over the widest possibletemperature range. Likewise, the base metal selected is preferably onethat can convert NO and NO₂ to N₂ across the widest possible temperaturerange and the widest range of NO/NO₂ ratios.

[0044] The advantage of the catalyst system of this invention is the useof a combination of a lean NOx trap and an NH₃—SCR catalyst. The use ofa lean NOx trap in the present system allows for much greater storage ofNOx, because the NOx breakthrough that would otherwise happen can becontrolled by the NH₃—SCR catalyst. Additionally, the use of a lean NOxtrap as part of this system allows for the operation of the engine atlean conditions for a longer time, and thus provides improved fueleconomy. If, for example, a three-way catalyst is used as the NOxstorage mechanism, NOx storage is significantly limited, as well as theproduction of ammonia. To maximize the reduction of emissions, athree-way catalyst must be operated at stoichiometric conditions.Accordingly, unless the three-way catalyst is run on the rich side 100%of the time, ammonia production is significantly less than for a typicallean NOx trap. As set forth above, the efficiency of a three-waycatalyst is compromised if it is operated at conditions other than atstoichiometric conditions. Thus the combination of a lean NOx trap andNH₃—SCR catalyst allows for significant NOx storage and ammoniaproduction and thus increases net NOx conversion.

[0045] In a preferred embodiment, the lean NOx trap and NH₃—SCR catalystconstitute alternating zones in a single substrate and/or a singlecatalytic converter can. This zoned design, as shown in three differentembodiments in FIGS. 4a-4 c, is believed to maximize the reactionbetween ammonia and NOx.

[0046] As illustrated in FIG. 4, three zoned catalyst system embodimentswere evaluated on a laboratory flow reactor. The total catalyst systemdimensions were held constant at a 1″ diameter and 2″ length. The firstsystem, labeled “4a”, had a 1″ long lean NOx trap followed by a 1″ longNH₃—SCR catalyst. In the second system, labeled “4b”, the catalystsamples were sliced in half to yield alternating ½″ long sections.Finally, in the third system, labeled “4c”, the same catalyst sampleswere further cut in half to yield ¼″ long sections, again of the leanNOx trap and NH₃—SCR catalyst technologies. It should be noted that eachtime the catalysts were sliced, as shown in “4b” and “4c”, the overalllength of the catalyst system was reduced slightly, approximately{fraction (3/16)}″ total. The alternating lean NOx trap and NH₃—SCRcatalyst zones can be created in a single substrate or the lean NOx trapand NH₃—SCR catalyst prepared, cut as desired and then placed adjacentone another in a single can. The zones are preferably formed in a singlesubstrate. However, cut substrates placed in alternating fashion alsoexhibit improved net NOx conversion.

[0047] Under the zoned catalyst designs shown in FIGS. 4a-4 c, wherealternating lean NOx and NH₃—SCR catalyst zones are provided, theammonia formed by the lean NOx trap is believed to be immediatelyadsorbed by the NH₃—SCR catalyst for use in the NOx conversion reaction.It is further believed that the greater the separation between the leanNOx trap and the NH₃—SCR catalyst, the greater chance there is for theammonia to be converted back into NOx. It is further believed thatoxygen is more abundant in the back of a catalyst substrate and thus theoxygen may be available to effectuate the unwanted conversion of theammonia back to nitrogen oxide. Accordingly, if the catalyst substrateis too long, there may be some undesired conversion that takes place;and thus in a preferred embodiment, the substrate is designed so thatammonia is available for immediate reaction with NOx.

[0048]FIGS. 5a-5 c illustrate laboratory reactor data of the threedifferent zoned catalyst system embodiments shown in FIGS. 4a-4 c. Thislaboratory data was obtained with the three catalyst systems operatingat a 250° C. inlet gas temperature and operating with 50 second lean and5 second rich cycles. Additionally, the inlet concentration of the NOxfeed gas was 500 ppm and the overall space velocity was 15,000 per hour.As illustrated in FIGS. 5a-5 c, with the use of a two-zoned catalystsystem as depicted in FIG. 5a, approximately 50 ppm of NO is emitted.This two-zone catalyst system resulted in a gross NOx conversion of 95%and a net NOx conversion of 66%. The four-zone catalyst embodiment,depicted as FIG. 5b, significantly reduced NOx emissions, well below the15 ppm range, to result in gross NOx conversion of 99% and a net NOxconversion of 86%. Finally, as illustrated by the eight zoned catalystembodiment, FIG. 5c, gross NOx conversion is 100% and net NOx conversionis 97.5%. The improvement comes from the reduction of N₂O, eliminationof the NH₃ breakthrough and reduction of NOx. Accordingly, as thecatalyst system is zoned down from 1″ sections to¼ sections, the testresults revealed an associated improvement in net NOx conversion.

[0049] As shown in FIGS. 5a-5 c, a zoned catalyst, with alternating leanNOx and NH₃—SCR catalysts in 1″ to ¼″ sections significantly improvesthe net NOx conversion from 66% to 97.5%. In addition, the gross NOxconversion is improved from 95% to 100%. In general, the improvement inthe net NOx conversion is the function of the elimination of the ammoniaslip, reduction in N₂O, and extra NOx reduction related to the NH₃ +NOxreaction on the NH₃—SCR catalyst. It is further believed that the dropin N₂O emissions is likely due to a higher fraction of the NOx reductionreaction proceeding on the NH₃—SCR catalyst rather than the lean NOxtrap. NOx reduction over a platinum-containing-lean NOx trap results inhigh levels of N₂O generation, whereas the NH₃—SCR catalyst has a highselectivity to nitrogen.

[0050]FIGS. 6a-6 c depicts laboratory data obtained using thethree-zoned catalyst embodiments originally shown in FIGS. 4a-4 c at a200° C. inlet gas temperature operating with a 25 second lean cycle anda 5 second rich cycle. As compared to FIGS. 5a-5 c, it should be notedthat shortening the lean time from 50 seconds, as used in FIGS. 5a-5 c,to 25 seconds, resulted in a substantial higher steady emission ofammonia—a fact which results in reduced net NOx conversion rates, ascompared to the data charted in FIGS. 5a-5 c. As can be seen in FIGS.6a-6 c, the use of smaller zoned sections from two zones to eight zones,and thus 1″ sections down to ¼″ sections, as illustrated in FIGS. 6a and6 c, improves the net NOx conversion from 50% to 81%. Again, thisimprovement is believed to come mainly from the reduction of ammoniabreakthrough and a small reduction in N₂O emissions. This lab data wasobtained with an inlet concentration of the NOx feed gas at 500 ppm andan overall space velocity at 15,000 per hour.

[0051] As set forth above, in the preferred embodiment, the lean NOxtrap washcoat and NH₃—SCR washcoat are combined in a single substraterather than placing the NH₃—SCR formulation downstream of the lean NOxadsorber as two separate catalyst substrates. Under this embodiment, thecatalyst formulations can be incorporated together by mixing or layeringthe washcoats on a substrate.

[0052]FIGS. 7a-7 c show three proposed washcoat configurationsincorporating the lean NOx trap and NH₃—SCR formulations into the samesubstrate. As shown in FIGS. 7a and 7 b, the first and second proposedconfigurations have the lean NOx trap and NH₃—SCR washcoat formulationson the bottom and top layer, respectively. It is believed that the toplayer could be a highly porous structure that allows better and fastercontact between the chemicals and gas phase and the active sites in thesecond layer. The third configuration, as shown in FIG. 7c, involves theuse of a one layer washcoat containing both lean NOx trap and NH₃—SCRwashcoat formulations. Under this third configuration, shown in FIG. 7c,the washcoat composition of the lean NOx trap and NH₃—SCR catalyst couldbe homogeneously or heterogeneously mixed. For a heterogeneously mixedcomposition, the formulation of the lean NOx trap and NH₃—SCR catalystare separated. However, they contact each other in varying degrees bycontrolling the size of the grain structures. The homogeneously mixedcomposition allows for a more intimate contact between the twoformulations and is thus preferred.

[0053] The invention also contemplates engineering such combinationswithin the pores of the monolithic substrate. An example of this isincorporating washcoat into porous substrates used for filtering dieselparticulate matter. Thus, this lean NOx trap/NH₃—SCR catalyst conceptcan be integrated into diesel particulate matter devices.

[0054] This very active SCR reaction of NOx and ammonia proceeds with orwithout oxygen present. Koebel et al. reports that the fastest SCRreaction involves equal molar amounts of NO and NO₂. NO and NO₂ thenreact with two NH₃ to yield N₂ in the absence of oxygen. In contrast,the lean NOx adsorber reaction of NOx plus CO is highly reactive only inan oxygen-free environment. In a lean NOx adsorber system, NOx isadsorbed during the lean cycle duration, NOx is not reduced.Accordingly, NOx reduction is limited to only the rich pulse timeduration. On the other hand, the lean NOx adsorber+NH₃—SCR catalystsystem allows for NOx reduction reaction to proceed during both the leanand rich time durations. Accordingly, ammonia as a reductant can beconsidered as a much more robust reductant than carbon monoxide.

[0055] As set forth above, the fastest SCR reaction involves equal molaramounts of NO and NO₂. Accordingly, FIG. 8 illustrates the impact ofvarying NO:NO₂ ratios after hydrothermal aging. FIG. 8 is a graph ofthree NH₃—SCR catalyst formulations over a wide NO:NO₂ range. In thelaboratory, it was possible to control the NO:NO₂ ratio entering thedownstream NH₃—SCR catalyst. Accordingly, the NO:NO₂ ratio entering theNH₃—SCR catalyst was solely dependent on the upstream lean NOx adsorber.In some cases, the majority of the feed NOx (especially NOx spikes) aremade up of mostly NO rather than NO₂. Accordingly, it is believed thatthe catalyst formulations of this invention will enhance reported netNOx efficiency—and thus the preferred catalyst is one that is capable ofoperating across the broadest range of NO:NO₂ ratios, and at a fullspectrum of temperature ranges.

[0056] In general, since NH₃—SCR catalysts do not contain preciousmetals, they are significantly less costly than a typical lean NOx trap.Accordingly, it is more cost effective to have an overall catalystsystem containing a lean NOx trap adsorber and an NH₃—SCR catalystsystem, rather than one that uses two lean NOx trap adsorbers.Additionally, the incorporation of both a lean NOx trap and NH₃—SCRwashcoat into a single substrate will significantly reduce substratecosts.

[0057] In another embodiment of this invention, NH₃ and NOx in anexhaust stream are reduced using a stoichiometric three-way catalystsystem. This three-way catalyst system has particular application forhigh speed/high flow rate conditions (i.e., USO6 conditions). Currently,three three-way catalysts are used for such high speed conditionapplications, wherein the third three-way catalyst is primarily directedto NOx removal for high speed/high flow rate conditions. Under thisalternate embodiment, the third three-way catalyst can be substitutedwith an NH₃—SCR catalyst to store NH₃ for reaction with NOx to improvenet NOx conversion, eliminate NH₃ emissions and reduce catalyst costs.

[0058] To improve net NOx and NH₃ reduction, the second three-waycatalyst can be modified to enhance the three-way catalyst's ability togenerate NH₃ emissions. To this end, in a preferred embodiment, thethree-way catalyst is designed to generate desirable NH₃ creation byusing platinum as the precious metal of the three-way catalyst, byplacing platinum on the outer layer of the three-way catalyst tomaximize the NO+H₂→NH₃ reaction. Likewise, the oxygen storage capacity(OSC) of the three-way catalyst can be removed to further promote thecreation of “desirable” NH₃. By doing so, the NH₃ purposely generatedduring rich operation can then be stored by the NH₃—SCR catalyst forsubsequent reaction with NOx emissions, and thereby control both NOx andNH₃ emissions under all operating conditions.

[0059] When a car is operated under rich conditions, the air/fuel ratiois less than 14.6, hydrogen is produced in the exhaust via the water-gasshift reaction: CO+H₂O→CO₂+H₂. The hydrogen that is produced then reactswith NOx as it passes over the precious metal surface to create“desirable” ammonia. The ammonia produced is then stored on an NH₃—SCRcatalyst to help reduce net NOx conversion. The reaction ofNOx+NH₃→N₂+H₂O can then take place on a separate NH₃ selective catalyst,capable of converting NO₂ and NO to N₂.

[0060] As shown in FIG. 9, a stoichiometric three-way catalyst/NH₃—SCRcatalyst system 10 is depicted, including a first three-way catalyst 14positioned in close proximity to the engine 12 to reduce cold startemissions. The second three-way catalyst 16 is modified as describedabove to enhance the ability of the second three-way catalyst 16 togenerate NH₃ emissions. Downstream of the second three-way catalyst 16is an NH₃—SCR catalyst 18 that functions to store NH₃ produced by themodified second three-way catalyst 16 for reaction with NOx emissions,to reduce both NOx and NH₃ emissions.

[0061] By substituting the third three-way catalyst as currently usedwith an NH₃—SCR catalyst and thereby eliminating the need for a thirdprecious metal containing catalyst, significant cost savings can beachieved.

[0062] It should further be noted that this invention also contemplatesthe use of a three-way catalyst, in combination with a lean NOx trap andan NH₃—SCR catalyst.

[0063] While the best mode for carrying out the invention has beendescribed in detail, those familiar with the art to which this inventionrelates will recognize various alternative designs and embodiments forpracticing the invention as defined by the following claims.

1. An emission control system for controlling NOx and NH emissions froman exhaust stream, the system comprising: a lean NOx trap incommunication with the exhaust stream for reducing NOx emissions; and aNH₃—SCR catalyst in communication with the exhaust stream for adsorbingNH₃, wherein the NH₃ adsorbed by the NH₃—SCR catalyst reacts with NOx inthe exhaust stream to improve the reduction of NOx and NH₃.
 2. Theemission control system of claim 1, wherein one or more alternatinglayers of the lean NOx trap and NH₃—SCR catalyst are provided in asingle catalytic converter shell.
 3. The emission control system ofclaim 1, wherein one or more alternating layers of the lean NOx trap andNH₃—SCR catalyst are provided in a single substrate.
 4. The emissioncontrol system of claim 1, wherein one or more alternating zones of thelean NOx trap and NH₃—SCR catalyst are provided in a single catalyticconverter shell.
 5. The emission control system of claim 4, wherein eachalternating zone of the lean NOx trap and alternating zone of theNH₃—SCR catalyst have a 141 length and 1″width.
 6. The emission controlsystem of claim 4, wherein each alternating zone of the lean NOx trapand alternating zone of the NH₃—SCR catalyst have a ½″ length and awidth of ½″.
 7. The emission control system of claim 4, wherein eachalternating zone of the lean NOx trap and alternating zone of theNH₃—SCR catalyst have a length of ¼″ and a width of ¼″.
 8. The emissioncontrol system of claim 1, wherein one or more alternating zones of thelean NOx trap and NH₃—SCR catalyst are provided in a single substrate.9. The emission control system of claim 1, wherein the lean NOx trapgenerates a sufficient quantity of NH₃ to force the reaction between NOxand NH₃, whereby NH₃ emissions are eliminated and net NOx conversionimproved.
 10. The emission control system of claim 1, wherein the leanNOx trap is optimized for NH₃ generation by removing oxygen storagecapacity of the lean NOx trap.
 11. The emission control system of claim1, wherein the lean NOx trap comprises a precious metal selected fromthe group consisting of platinum, palladium, rhodium and combinationsthereof; and a NOx storage material selected from the group consistingof alkali metals, alkali earth metals, rare earth metals andcombinations thereof.
 12. The emission control system of claim 1,wherein the lean NOx trap comprises platinum.
 13. The emission controlsystem of claim 1, wherein the lean NOx trap comprises a composite ofcerium and zirconium.
 14. The emission control system of claim 1,wherein the lean NOx trap comprises one or more materials for NH₃generating and NOx storage.
 15. The emission control system of claim 1,wherein the NH₃—SCR catalyst comprises one or more NH₃ adsorbingmaterials, wherein the NH₃ adsorbing materials are capable of convertingNOx and NH to nitrogen.
 16. The emission control system of claim 1,wherein the NH₃—SCR catalyst comprises a base metal and a supportselected from the group consisting of alumina, silica titania, zeoliteand their combinations.
 17. The emission control system of claim 1,wherein the NH₃—SCR catalyst comprises a metal selected from the groupconsisting of Cu, Fe and Ce and a zeolite.
 18. The emission controlsystem of claim 1, wherein the lean NOx trap and NH₃—SCR catalyst areplaced in a single catalytic converter shell.
 19. The emission controlsystem of claim 1, wherein the NH₃—SCR catalyst is separate from anddownstream from the lean NOx trap.
 20. An emission control system forcontrolling NOx and NH₃ emissions from an exhaust stream produced by thecombination of an air/fuel mixture in an internal combustion engine, thesystem comprising: a lean NOx trap in communication with the exhauststream for NOx reduction wherein the lean NOx trap comprises a lean NOxtrap formulation which includes one or more NOx storage and NH₃generating materials; a NH₃—SCR catalyst in communication with theexhaust stream for adsorbing NH₃, wherein the NH₃—SCR catalyst comprisesa NH₃—SCR catalyst formulation which includes one or more NH₃ adsorbingmaterials; and wherein the lean NOx trap formulation and the NH₃—SCRcatalyst formulation are placed on one substrate.
 21. The emissioncontrol system of claim 20, wherein a layer of the lean NOx trapformulation and a layer of the NH₃—SCR catalyst formulation are placedon the substrate to form a two-layer washcoat.
 22. The emission controlsystem of claim 20, wherein the lean NOx trap formulation and theNH₃—SCR catalyst formulation are homogeneously mixed to form a singlewashcoat layer on the substrate.
 23. The emission control system ofclaim 20, wherein the lean NOx trap formulation and the NH₃—SCR catalystformulation are heterogeneously mixed to form a single washcoat layer onthe substrate.
 24. An emission control system for controlling NOx andNH₃ emissions from an exhaust stream produced by the combination of anair/fuel mixture in an internal combustion engine, the systemcomprising: a lean NOx trap in communication with the exhaust stream; aNH₃—SCR catalyst in communication with the exhaust stream for adsorbingNH₃, wherein the NH₃ adsorbed by the NH₃—SCR catalyst reacts with NOx inthe exhaust stream to improve NOx and NH₃ reduction; and wherein thelean NOx trap and NH₃—SCR catalyst are provided in one substrate. 25.The emission control system of claim 24, wherein one or more alternatingzones of the lean NOx trap and NH₃—SCR catalyst are provided, each zonehaving a 1″ width.
 26. The emission control system of claim 24, whereinalternating zones of the lean NOx trap and NH₃—SCR catalyst areprovided, each zone having a ½″ width.
 27. The emission control systemof claim 24, wherein alternating zones of the lean NOx trap and NH₃—SCRcatalyst are provided, each zone having a ¼″ width.
 28. The emissioncontrol system of claim 24, wherein the lean NOx trap and the NH 3—SCRcatalyst are placed in one or more alternating layers in the substrate.29. An emission control system for controlling NOx and NH₃ emissionsfrom an exhaust stream produced by the combination of an air/fuelmixture in an internal combustion engine, the system comprising: a leanNOx trap in communication with the exhaust stream; a NH₃—SCR catalyst incommunication with the exhaust stream for adsorbing NH₃, wherein the NH₃adsorbed by the NH₃—SCR catalyst reacts with NOx in the exhaust streamto improve NOx and NH₃ reduction; and wherein the lean NOx trap andNH₃—SCR catalyst are provided in a single catalytic converter shell. 30.An emission control system for controlling NOx and NH₃ emissions from anexhaust stream produced by the combination of an air/fuel mixture in aninternal combustion engine, the system comprising: a lean NOx trap incommunication with the exhaust stream for NOx reduction, to provide aNOx reducing exhaust stream including NOx and NH₃; and a NH₃—SCRcatalyst in communication with the exhaust stream for adsorbing NH₃,wherein the NH₃ adsorbed by the NH₃—SCR catalyst reacts with NOx in theNOx reduced exhaust stream to improve the reduction of NOx and NH₃. 31.The emission control system of claim 30, wherein the lean NOx trapgenerates a sufficient quantity of NH₃ to force the reaction between NOxand NH₃, whereby NHemissions are eliminated and net NOx conversionimproved.
 32. The emission control system of claim 30, wherein the leanNOx trap is optimized for NH₃ generation by removing oxygen storagecapacity.
 33. The emission control system of claim 30, wherein the leanNOx trap comprises a precious metal selected from the group consistingof platinum, palladium, rhodium and combinations thereof; and a NOxstorage material selected from the group consisting of alkali metals,alkali earth metals, rare earth metals and combinations thereof.
 34. Theemission control system of claim 30, wherein the lean NOx trap comprisesplatinum.
 35. The emission control system of claim 30, wherein the leanNOx trap comprises a composite of cerium and zirconium.
 36. A method ofcontrolling NOx and NH₃ emissions from an exhaust stream produced by thecombination of an air-fuel mixture in an internal combustion engine,comprising: providing a lean NOx trap in communication with the exhauststream; and providing an NH₃—SCR catalyst in communication with theexhaust stream for adsorbing NH₃, wherein the NH₃ adsorbed by theNH₃—SCR catalyst reacts with NOx in the exhaust stream.
 37. A catalystsystem for controlling diesel particulates, comprising: a poroussubstrate, including a washcoat containing lean NOx trap and NH₃—SCRcatalyst formulations, wherein the porous substrate filters dieselparticulates.
 38. A method of controlling diesel particulates from adiesel exhaust stream comprising: providing a porous substrate;incorporating a washcoat comprising lean NOx trap and NH₃—SCRformulations into the porous substrate; and passing the diesel exhauststream through the porous substrate to filter diesel particulates. 39.An emission control system for controlling NOx and NH₃ emissions from anexhaust stream produced by the combination of an air/fuel mixture in aninternal combustion engine, the system comprising: a three-way catalystin communication with the exhaust stream to reduce NOx emissions andproduce NH₃, wherein the three-way catalyst comprises platinum on anouter surface of the three-way catalyst to optimize the formation ofNH₃, and wherein the three-way catalyst is further optimized for NH₃generation by removing oxygen storage capacity of the three-waycatalyst; and an NH₃—SCR catalyst in communication with the exhauststream for adsorbing NH₃, wherein the NH₃ adsorbed by the NH₃—SCRcatalyst reacts with NOx in the exhaust stream to improve the reductionof NOx and NH₃.